1. Technical Field
This invention relates generally to electrostatographic copiers/printers, and more specifically the procedure for separating receiver sheets from the image member thereof. As used herein, the term "copiers/printers" refers to copiers, printers, and copier-printer combinations.
2. Background Art
In a typical electrostatographic process, an electrostatic charge pattern having an image-wise configuration corresponding to information to be reproduced is formed on the surface of an image member. The charge pattern is developed by applying developer material to the pattern to form a transferable image on the image member. The developer material includes for example, thermoplastic pigmented marking particles which are attracted to the charge pattern by electrostatic forces.
Transfer of this image from the image member to a receiver sheet is accomplished by spraying negative charge on the receiver sheet using a transfer device such as a semi-conductive roller biased at a high potential. This negative charge on the receiver sheet after transfer either pairs with the positive charge of the toner on the image member or, in those areas which do not have an appreciable deposit of toner, couples mainly with its induced positive-polarity image charge in the conductive layer of the image member (FIG. 1). The negative charge in the non-image areas is beneficial in the sense that it electrostatically tacks the receiver sheet to the image member to better control its passage between the transfer station and the detack station, where the receiver sheet is separated from the image member before the image is permanently fixed to the sheet to form the reproduction.
Separation of the receiver sheet from the image member is referred to as the "detack" process. Detack is effected by means of a mechanical stripping pawl, by applying an electrical charge to remove any charge build up on the receiver sheet to lessen the force of attraction acting between the sheet and the image member, or both.
When separating a receiver sheet from the image member, the detack charger reduces the overall level of negative charge to a value where the receiver sheet is less tightly bound such that it can be more easily removed at the detack roller. The remaining negative charge, however, needs to still be great enough to hold the toner to the receiver sheet during and after separation of receiver sheet and image member. This requires a balanced situation between failure to detack and blow-off (transfer of toner back to the image member).
In most circumstances detack is successful, especially when there is appreciable image content or when there is a significant amount of toner near the lead edge of the receiver sheet. In this case, the toner more or less functions as a release agent, acting both electrically by coupling with the negative receiver charge, and physically as a spacer between the receiver sheet and image member. This has the net effect of reducing the adhesive and electrostatic binding force between receiver sheet and image member.
Some copier or printer jobs, however, require that receiver sheets be transported through the normal paper path even though an image is not written on the portion (frame) of the image member associated with that receiver sheet. Examples are spacer sheets to be inserted between transparencies, cover sheets for reports, separator sheets between chapters, etc. In this case, machine logic and software would not normally activate the development station because development need not take place. In order to keep the electrostatographic process and associated software as simple as possible but at the same time control the fate of the receiver sheet, the receiver sheet and image member are nevertheless allowed to experience the electric fields established by the transfer and detack subsystems, even though there is no toned image on the image member.
It has been found, however, when carrying out the above sequence of events, that blank receiver sheets do not detack as reliably as imaged sheets. When a blank receiver sheet is fed through the system and there is no toner on the image member, detack failures commonly occur. In this case, almost all of the transfer charge is coupled to any induced charge in the conductive layer of the image member. There is essentially no toner between receiver sheet and image member to reduce this binding force (FIG. 2). In this case, the normal amount of charge from the detack charger plus the beam strength of the receiver sheet is often insufficient to release the receiver sheet from the image member. The inability to detack reliably is even more apparent with lighter-weight receiver sheets having still lower beam strength.
One way of ensuring detack of blank receiver sheets is to increase the detack charger output and essentially neutralize all of the charge on the receiver sheet. This, however, would require a programmable power supply and the means to predict the arrival of blank receiver sheets.